Known polarizing films include a uniaxially stretched PVA film on which iodine and/or dichroic dye(s) are adsorbed. A PVA film employed in this polarizing film as a base generally comprises PVA of a low degree of polymerization, such as 2,000 or below. This polarizing film has been widely employed, for example, in liquid crystal displays of, e.g., pocket electric calculators, watches, word processors, liquid crystal printers, liquid crystal color TV's, various instruments and automotive instrument panels. Further, it is inserted into laminated glass to thereby form an anti-glare film for, e.g., sunglasses or ski goggles. Although this polarizing film exhibits an excellent polarizing coefficient, it has a poor water resistance and moist heat resistance and lacks heat resistance since it contains a hydrophilic polymer as the base. Therefore, this polarizing film frequently suffers from the deterioration of optical properties, such as a decrease in the polarizing coefficient, when exposed to high temperature and/or humidity. With the development of the electronic industry, the application range of liquid crystal displays has been more and more extended. Accordingly, a polarizing film is urgently required which has a high water resistance, heat resistance and moist heat resistance in addition to excellent optical properties such as a high polarizing coefficient and transmittance.
Under these circumstances, there has been proposed a uniaxially stretched polyester film of improved heat resistance and moist heat resistance which is obtained by incorporating a dichroic dye into a polyester, melt-extruding the resulting mixture and then stretching the same (cf. JP-A-58-68008, JP-A-58-124621, JP-A-60-125804, JP-A-61-65202, JP-A-62-141503 and JP-A-62-145204) (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Although this polarizing film is superior in heat resistance and moist heat resistance to a conventional PVA polarizing film, the polarizing coefficient thereof is still insufficient. For this reason, it has been seldom used in practice, compared with PVA polarizing films.